During the past decade, a variety of horizontal flow-through, coil planet centrifuges have become available that are particularly suitable for performing preparative countercurrent chromatography (CCC, hereinafter). Examples of such devices include my U.S. Pat. Nos. 4,058,460 disclosing a horizontal flow-through, coil planet centrifuge without rotating seals, in which the planetary motion allows both gravitational and centrifugal force fields to act in a time-varying manner to provide highly efficient mixing of two solvent phases in the coil for chromatographic separations of constituents therebetween; No. 4,324,661 claiming methods for centrifugal CCC extraction of separable components from a liquid sample comprising two respective solvent phases of different viscosity and density through opposite ends of an axially rotating column wherein a liquid sample is introduced at a middle portion of the column and a selected separable component of the sample is collected from one of the two ends of the column; and No. 4,487,693 disclosing a multilayer coil countercurrent chromatograph with adjustable revolutional radius to enable an adjustment of the so-called ".beta." value of the device to make it substantially greater than 1. .beta. may be defined in terms of the geometry of such systems as the ratio of the radius characterizing rotation of the tubing coil about an axis of a holder to the radius at which the axis of this holder revolves around a principal/stationary axis of the system to generate the compound synchronous motion to which fluid flowing through the coil is subjected during use.
Experience with such devices, in which the tubular coil through which the fluid materials are flowed undergoes a synchronous planetary motion about a stationary axis to provide efficient mixing of the two solvent phases, teaches that having the system rotate and revolve about horizontal axes is particularly helpful. Because such a system permits stable retention of low interfacial tension two-phase solvent systems, such apparatus has been extensively used for separations of peptides and other polar compounds with hydrophilic butanol solvent systems. This is described in considerable detail in, for example, Versatile Coil Planet Centrifuge for Performing Countercurrent Chromatography: Comparative Studies on Performance on Three Types of Columns, Sandlin, J.S. and Ito, Y., Journal of Liquid Chromatography, 11(1), 55-77 (1988). This article teaches three different column configurations, i.e., coaxial multilayer coil, toroidal coil form, and eccentric dual layer coils mounted around the holder. Among these the concentric multilayer coil arrangement, although it can hold the maximum volume of the solvent, fails to retain the polymer phase systems. The eccentric coil arrangement which relates to the present invention, on the other hand, provides stable retention for all the solvent systems including viscous polymer phase systems.
As is amply disclosed in the references listed hereinabove, and as is well understood by persons skilled in the relevant art, rotation of a helically wound tubular coil about an axis of rotation revolving around a stationary axis creates certain highly desirable effects on fluids inside the coil that are advantageously exploited for chromatographic studies. Revolution of the coil contributes to stabilizing one of the two phases being flowed therethrough, i.e., that phase which may be identified as the "stationary phase", while rotation of the coil about the holder axis produces mixing with the other or "mobile phase" to promote partition processing of solutes therefrom. In order to achieve the best results, these two effects should be optimized to yield a satisfactory retention level of the stationary phase and at the same time generate an efficient mixing of the two phases in the coil. In practice, after the apparatus has been operated for some time, a hydrodynamic equilibrium is obtained in which the stationary phase is retained in the coil and the mobile phase is introduced at an inlet of the coiled tubing elutes through the coil. Samples of materials introduced in solution in the mobile phase are partitioned between the two phases and are separated chromatographically according to their partition coefficients.
As will be immediately apparent, the formation of such a hydrodynamic equilibrium is dependent upon a first set of variables inherent to the solvent system used, e.g., interfacial tension, tube wall affinity, density differences, and the viscosities of the materials constituting the two phases, as well as on a second set of variables related to the physical aspects of the apparatus, e.g., the rotational speed at which the apparatus is operated, the internal diameter of the tubing used, the flow rate of the mobile phase, the relative directions in which the rotation and revolution of the coiled tubing are effected, and the .beta. value of the apparatus.
Experimental studies have shown, however, that the formation of this equilibrium is secondary to the acceleration field to which the coiled tubing is subjected. Structures to avoid sealing problems that would otherwise be encountered due to twisting of tubing enabling fluid flow to the rotating coils are described generally in the cited references, especially in my U.S. Pat. No. 4,058,460, which is incorporated herein by reference for its disclosure of a planetary gear system for avoiding flow tube twisting. In summary, a typical structure of this type causes a column holding one or more helical coils of tubing to revolve once with respect to a stationary, central, preferably horizontal axis of the apparatus while simultaneously rotating with the same angular velocity about the rotational axis that is revolving around the stationary axis.
Such an arrangement obviates sealing problems, twisting of the tubing and the like, and generates an acceleration field that varies with the value of .beta., the radial span of the tubing coil, and the speed of rotation. Although the centrifugal force field is always directed radially outward, what is radially outward for each point in the coil subjected to synchronous planetary motion varies with time and its instantaneous location and, furthermore, is coupled with the ever present gravitational field acting towards the center of the earth. In effect, therefore, the net acceleration experienced by fluid at any point in the coil subjected to synchronous planetary motion as indicated is constantly varying in time.
The acceleration field also generates centrifugal forces on the columns or formers about which the liquid filled tubing is coiled. While higher partition efficiencies may be obtained by providing longer lengths of tubing coil, the very length of such an element will lead inevitably to potentially unacceptable physical deformation due to centrifugal forces acting thereon during use. One solution clearly is to shorten the overall length of the helical coil, and one way to counter a corresponding diminution of the overall length of tubing and the net volume of liquid subjected to the acceleration field within the coil is to form a multilayer coil. My, U.S. Pat. No. 4,487,693 discloses the use of a single multilayer coil in an apparatus in which a physical central shaft is eliminated to obtain values of .beta. greater than 1. However, the need for longer tubing length and liquid volume within the acceleration field, while limiting centrifugal deformation of the tubing coil itself, remains a factor that calls for further enhancement.
A need, therefore, exists for a device of the type generally described hitherto that permits seal-free provision of inflows and outflows of selected fluids to subject them to strong gravitational-centrifugal acceleration fields in synchronous planetary motion, with limited physical deformation of the apparatus, while providing an extensive length of tubing to conduct mixing fluid flow and of a significant volume of liquid subjected to a strong time-varying acceleration field.